Title of Invention

A PROCESS FOR PRODUCING ETHANOL IN A CORYNEFORM BACTERIUM

Abstract

A process for highly efficiently producing ethanol at a high productivity by using a coryneform bacterium, which has been transformed by DNA containing a gene expressing a pyruvate decarboxylase activity and, if desired, a gene expressing an alcohol dehydrogenase activity under a regulatory sequence allowing the expression, under ethanol production conditions wherein this bacterium does not substantially proliferate.

Full Text

DESCRIPTION PROCESS FOR PRODUCING ETHANOL BY USING RECOMBINANT CORYNEFORM BACTERIUM FIELD OF THE INVENTION The present invention relates to a process for producing ethanol. Particularly, the present invention relates to a process of producing ethanol wherein sugars such as glucose are used as raw material, and more particularly, a process for highly efficiently producing ethanol at a high productivity consisting of using a coryneform bacterium which has been transformed by a DNA containing gene expressing pyruvate decarboxylase (referred to hereinafter as PDC) activity and, if desired, a gene expressing alcohol dehydrogenase(referred to hereinafter as ADH) activity under a regulatory sequence allowing the expression, and fermenting a raw material, sugars such as glucose, under ethanol production conditions wherein this bacterium does not substantially proliferate to produce ethanol, which is then collected. BACKGROUND ART Up to now, ethanol has been prepared by chemical synthesis via ethylene from fossil resources such as coal and petroleum, or a fermentation of sugars from biomass resource such as plant by a microorganism such as yeast or bacteria. Among these, a process for producing ethanol by fermentation using a renewable biomass resource in light of energy resource or environmental problems has been noticed. A process for producing ethanol by a conventional large-scale industrial fermentation is a process by fermenting starch or sugars from various biomass resources, namely, a technology based on a brewage for potable ethanol. However, the use of a yeast as a fermentation microorganism results in slow rate of ethanol production and also difficulties such as complicated fermentation control because of the necessity of aeration, even though the fermentation is performed under anaerobic condition. It is well known to use a strain belonging to Zymomonas as a fermentation microorganism (Japanese Patent Publication No. H7-59187). It is recognized that a fermentation process by using Zymomonas leads to increased rate of ethanol production compare to that by using a yeast. Improvement of ethanol production efficiency by biotechnological modification of various microorganism species has been proposed (Japanese Patent Publication Nos. H5-502366, H6-504436, and H6-505875). In the above tebhnologies, 2 genes from Zymomonas mobilis, one encoding for PDC activity (which catalyses the conversion of pyruvic acid in the glycolytic pathway to acetoaldehyde) and another ADH activity (which catalyses the conversion of acetoaldehyde to ethanol) were inserted into host (enteric bacteria such as Escherichia col i or Erwinia chrysanthe) under a regulatory sequence allowing for their expression. Consequently, ethanol fermentation with high productivity was achieved by using such transformants. According to the abovementioned technologies, improvement in ethanol productivity owes to the high growth and high cell density of the transformants in the fermentor. Such fermentation involving cell growth presents many inefficiencies including low conversion efficiency of sugar material to ethanol because the sugar material is used as an energy supply source for growth of said microorganism, low ethanol production rate during the period until the microorganism reaches stationary growth phase and complications in fermentor control due to the change of density of the microorganism accompanying the cell growth thereof. Another technological problem is separation of toxic substances when Escherichia coli used as a host cell. Escherichia coli is succeptible to bacteriolysis leading to possible contamination of fermentation'products with toxic intracellular protein. From this point of view, improvement of host microorganism to be transformed is desirable. PROBLEM TO BE SOLVED BY THE INVENTION The present invention provides a new process for producing ethanol by using biomass resources as raw material for sugars, wherein better excellent technology for efficiently producing ethanol at a high productivity has been accomplished and wherein various problems pointed out in the foregoing prior art have been solved. ■ MEANS TO SOLVE THE PROBLEM The present inventors have made extended studies to solve the problems mentioned above, and have found that efficient production of ethanol at a high productivity can be achieved by using a coryneform bacterium which has been transformed with a gene expressing PDC activity, and if desired, a gene expressing ADH activity under a regulatory sequence allowing for expression, and then, performing fermentation under ethanol production conditions wherein the transformed coryneform bacterium does not substantially proliferate, whereby the present invention has been accomplished. One of the characteristics of the present invention is that a host bacterium to be transformed is a coryneform bacterium and that ethanol is prepared under conditions wherein the transformed coryneform bacterium does not substantially proliferate. EMBODIMENT OF THE INVENTION A gene expressing ADH activity and a gene expressing PDC activity from various microorganisms can be used in the present invention. Although each of the genes may be derived from different mcroorganisms, Zymomonas mobilis can be preferably used. Specifically, the following gene, which have been already cloned and sequenced, can be used. Genes expressing ADH activity Origin Zymomonas mobilis: ADHI gene (ace. NO.M32 1 00) (J . Bacteriol. vol. 172, 2491-2492(1990)) ADHII gene (ace. NO.X1 7065, M1 5394) (J. Bacteriol. vol. 169,2591-2597(1987))0 Origin Saccharomyces cerevisiae: ADH1 gene (J. Biol. Chem. vol.257,301 8-3025(1 982)) ADH2 gene (J.Biol.Chem.vol.258,2674-2682(1983)) ADH3 gene (Nature,vol.387,90-93(1 997)) ADH4 gene (ace. NO.X05992) (Mol. Gen. Genet, vol. 209,374-381(1987)) ADH5 gene (EMBO J.vol. 1 3,5795-5809(1 994)) Origin Sinorhizobium meliloti: ADH gene (Biochim. Biophys.Acta vol. 1 384,1 97-203 (1998)) Origin Salmonella typhimurium: ADH gene (J. Bacteriol.vol. 1 81 (1 7),531 7-5329(1 999) Origin Mycobacterium tuberculosis: ADH gene (Nature,vol.393,537-544(1 998)) Origin Esherichia coli: ADH gene (DNA Res.vol.3,1 37-1 55(1 996))„ Genes expressing PDC activity Origin Zymomonas mobilis: PDC gene (J. Bacteriol.vol. 1 70,331 0-331 3(1 988)) Origin Saccharomyces cerevisiae: PDC1 (acc.NO.X77316) (Nucleic Acids Res.voM4, 893-63-8977(1986)) PDC6 (acc.NO.X66843,X55905) (J. Bacteriol. vol. 173, 7963-7969 (1991)) PDC5 (acc.NO.X15668) PDC2 (acc.No.X65608) (Mol. Gen. Genet. vol241 ,657-666(1993)) Origin Bacillus subtilis: pdhA gene /pdhB gene (ace. NO. AF01 2285) (J. Bacteriol. vol.172, 5052-5063(1990)) Origin Thiobacillus ferrooxidans: pdhA gene /pdhB gene (acc.NO. U81 808)(Microbiology, vol.142, 2543-2548(1996)). It is necessary for the two genes to be under a regulatory sequence in order to express their activity, although they are not necessarily required to be under a common regulatory sequence. They may be under separate regulatory sequences and in some cases on a different plasmids and at a different sites on a chromosome. "Under a regulatory sequence" as used herein means that an intended gene can autonomously replicate by, for example, collaboratively acting with a promoter, inducer, operator, ribosome binding site and transcription terminator etc. A host microorganism transformed according to the present invention is a coryneform bacterium. The coryneform bacterium is a Gram-positive bacterium and different from a Gram-negative bacterium such as E. coli which is used as a host bacterium in the aforementioned prior art. It is noteworthy that USP5,482,846 and USP5,916,787 disclosed a process for transforming a Gram- positive bacterium by a gene expressing ADH activity and PDC activity. However, the Gram-positive bacteria as used therein are Bacillus, Lactobacillus, Fibribacter, Ruminococcus, Pediococcus, Cytophaga, Cellulomonas, Bacteroides, Clostridium, Bacillus subtilis, and Bacillus polymyxa, but it is not taken into account that a coryneform bacterium can be used as the host cell to be transformed. One of the major characteristics of the present invention is that a coryneform bacterium, which does not originally have a function of ethanol production from pyruvic acid, is selected as the host to be transformed. Use of coryneform bacterium transformed according to the present invention, enabled us to prepare ethanol under conditions in which no substantial cell growth was observed, in contrast to the usual process for producing ethanol by fermentation with yeast, Zymomonas or enteric bacteria such as E. coli transformed according to the aforementioned prior art. Thus, by producing ethanol without cell growth, many technological problems resulting from cell growth as described above, are overcome. This has a significant value as a practical industrial production technology. The present inventors have discovered that such a technology can be achieved by using a coryneform bacterium. Substantial suppression of growth of coryneform bacterium can be achieved by subjecting the aerobic bacterium to anaerobic conditions, or restricting the essential nutritional requirement, biotin (J. Industrial. Microbiol. vol.5,289-2 94.(1990)). In addition, biotechhological regulation of a function of a gene regulating growth would be an effective means. Although ethanol fermentation under anaerobic condition has already been accomplished, the technology emphasizes or is accompanied by cell growth. On the contrary, the present invention is quite different to the extent that the present invention is intended to suppress the substantial growth of a coryneform bacterium in an ethanol reaction vessel. The aforementioned prior art discloses that both anaerobic and aerobic conditions lead to an equivalent growth of a transformant and productivity of ethanol, which differs from the present invention in technological idea or content. An essential element of the present invention is to introduce into a coryneform bacterium a DNA containing a gene expressing PDC activity and, if required, a gene expressing ADH activity under a regulatory sequence allowing for expression. In this process, it is essential to introduce a gene expressing PDC activity, but not essential to introduce a gene expressing ADH activity. This is because usually a coryneform bacterium is deemed to have neither genes expressing the foregoing respective enzymes but it is uncertain regarding ADH. Thus, it cannot be denied that certain coryneform bacteria have the gene expressing ADH enzyme activity. In this case, it is basically enough to use only a gene expressing PDC enzyme activity as the gene to be introduced. However, both genes are preferably introduced to effect higher transformation even if the coryneform bacterium used as a host originally has an ability to produce ADH. There is no limitation as the type of aerobic coryneform bacteria transformed with the gene expressing PDC activity and, if desired, the gene expressing ADH activity, as long as it is a coryneform bacterium which is able to growth under usual aerobic conditions. Examples thereof are Corynebacterium, Brevibacterium, Arthrobacter, Mycobacterium, and Micrococcus. And more particularly, examples of Corynebacterium are Corynebacterium glutamicum ATCC13032, ATCC13058, ATCC13059, ATCC13060, ATCC13232, ATCC13286, ATCC13287, ATCC13655, ATCC13745, ATCC13746, ATCC13761, ATCC14020, and ATCC31831. Examples of Brevibacterium are Brevibacterium lactofermentum ATCC13869, Brevibacterium flavum MJ-233(FERM BP-1497) and MJ- 233AB-41 (FERM BP-1498), Brevibacterium ammoniagenes ATCC6872. Examples of Arthrobacter are Arthrobacter globiformis ATCC8010, ATCC4336, ATCC21056, ATCC31250, ATCC31738 and ATCC35698. As a transformation, there are processes for recombining a chromosome of a host bacterium with both genes, or a process to use a recombinant plasmid wherein both genes have been incorporated into a plasmid which can autonomously replicate in a host bacterium. A plasmid vector used for such purposes may be one containing a gene with an autonomous replication function in a coryneform bacterium. As specific example, pAM330(Agric. Biol.Chem. vol. 48,2901-2903 (1984) and Nucleic Acids Symp Ser. Vol. 16,265-267 (1985)) (from Brevibacterium lactofermentum 225), pHM1 51 9(Agric. Biol.Chem.vol.48, 2901-2903(1984) (from Corynebacterium glutamicum ATCC13058), pCRY30(Appl. Environ. Microbiol, vol. 57, 759-764(1991)), pEKO, pEC5, pEKExl (Gene vol.102, 93-98 (1991)), and pCG4 (J. Bacteriol. Vol.159, 306-31 1(1984))(from Corynebacterium gluatmicum T250). Construction of a plasmid used for transformation of coryneform bacteria in the present invention can be effected, for example, in the case of a gene from Zymomonas mobilis being used, by linking a regulatory sequence such as an appropriate promoter and terminator with Dral-Dral 1.4 kb gene fragment containing an intact ADH gene (J. Bacteriol.vol. 1 69,2591-2597( 1 987)), and Dral- Dral 1.8 kb containing an intact PDC gene (J. -Bacterid, vol.169,949-954(1987)) respectively, which is then inserted at an appropriate restriction site of any one of a piasmid vector as exemplified above. An example of a promoter to express ADH gene and PDC gene in the aforementioned recombinant piasmid is, but not limited to, the one which is originally carried by a coryneform bacterium. It may be any base sequence which has a function to initiate the transcription of ADH gene and PDC gene. An example of a terminator which is put in the downstream of ADH gene and PDC gene under a regulatory sequence is, but not limited to, the one vfchich is originally carried by a coryneform bacterium. It may be, for example, any base sequence having a function to terminate the transcription of ADH gene and PDC gene, such as the terminator for tryptophan operon from E. coli. In the present invention, cultivation of the coryneform bacterium, which is performed prior to introduction of a piasmid vector containing such as the intended gene into the coryneform bacterium, can be performed under usual aerobic conditions. A medium used for cultivation of usual microorganism can be used as culture medium. For example, to a general medium containing natural nutrients such as meat extract, yeast extract, and peptone etc., solution of inorganic salt such as ammonium sulfate, potassium phosphate, and magnesium sulfate is added, if necessary. A process for introducing a plasmid vector containing the intended gene after the foregoing cultivation includes, but is not limited to, electroporation and the CaCI2 method as long as such process enables introduction of a gene into a coryneform bacterium. In an embodiment thereof, for example a known method can be used for an electrical pulsation (Agric.Biol.Chem.vol.54, 443-447 (1990), Res. Microbiol, vol. 144,181-185(1 993)). As for the introduction of the intended gene into a chromosome, a similar method is available, for example, technology such as that described in DNA sequence vol.3,303-310 (1993) is available. A method for selecting the transformed coryneform bacteria employs antibiotic resistance, whereby a gene encoding for the said resistance is introduced into a plasmid vector or a chromosome containing the intended gene, and then plating the transformed coryneform bacterium, onto a plate containing an appropriate concentration of the said antibiotic. As an embodiment thereof, for example a method as described in Agric. Biol. Chem. vol.54,443-447 (1990), Res. Microbiol, vol. 144, 181-185 (1993) is available. Basically, in order to use an aerobic coryneform bacterium for the process of preparing ethanol of the * present invention, a large amount of transformed coryneform bacteria must be cultured under usual aerobic conditions first. The present cultivation can be performed in a similar manner as a cultivation of a coryneform bacterium prior to transformation. Transformed coryneform bacteria of the present invention which were cultured under aerobic conditions are harvested by centrifugation, membrane filtration or chemically treated (for example, immobilization with carrageenan) in order to be used for a subsequent reaction to produce ethanol. For the reaction to produce ethanol, an aqueous solution containing the appropriate inorganic salt or buffer is preferably used. To the aqueous solution, sugars, such as glucose which is material for ethanol production, are added. The reaction to produce ethanol is performed under conditions wherein growth of the transformed aerobic coryneform bacterium is substantially suppressed. Various methods, as described above, can be employed to substantially suppress the growth, and it is adequate to put the transformed aerobic coryneform bacterium under anaerobic condition. A system for reaction to produce ethanol may be batchwise or continuous reaction, and preferably continuous in view of achieving high productivity. Use of continuous reaction system in the process of the present invention does not lead to substantial growth of the coryneform bacterium and therefore the operation control thereof is easier than that of a conventional method, which is accompanied by cell growth, Sugars used as raw material for ethanol production include, preferably, but are not limited to, glucose which leads to rapid ethanol production. "Anaerobic conditions" as used herein means any conditions resulting in lowered concentration of dissolved oxygen in aqueous solution, provided that a trace amount of the dissolved oxygen should be allowed as long as growth of the transformed coryneform bacterium is substantially inhibited. This condition is achieved by, for example, carrying out the reaction without aeration in a sealed container, or performing the reaction while supplying an inert gas such as nitrogen. The temperature for ethanol production is usually 15°C-45°C, and preferably 25°C-37°C. pH during the reaction is adjusted to the range of 5-9, and preferably 7-8. The coryneform bacterium used for ethanol production in the present invention can be used at a very high concentration such as 1 g/l-1500 g/l because it is not substantially growing during the reaction. In ethanol production of the present invention, the cells are not substantially growing during the reaction, and raw material sugars are not consumed as a source of nutrition for growth. In addition, no need for induction period such as the exponential growth phase for providing a high-density culture of bacteria means that ethanol production with high density (high concentration) of cells from the early stage of reaction is possible, thereby highly efficient ethanol production at a high productivity can be achieved. Ethanol prepared according to the process as stated above is isolated from the reaction and purified, if necessary, and utilized as fuel ethanol or industrial raw chemical material. The following examples illustrate the present invention in detail, but should not be deemed to limit the scope of the invention. EXAMPLES Example 1 Cloning of a DNA containing ADH gene and PDC gene fragments from Zymomonas mobilis ATCC29191. (A) Extraction of total DNA from Zymomonas mobilis ATCC29191: To 1L of a growth medium [Composition:20g of Glucose, 5 g of Yeast Extract and 1000ml of distilled water], a platinum loopful of Zymomonas mobilis was inoculated. It was then anaerobically cultured at 30 °C till at late exponential growth phase, and harvested. Bacterial cell obtained were suspended at the concentration of 10mg/ml to 15ml of a solution containing 10mg/ml lysozyme, 10mM NaCI, 20mM Tris buffer(pH8.0) and 1mM EDTA-2Na(the concentration of each components shows the final concentration). Then, protease K was added at the final concentration of 100 pg/ml, and heated at 37°C for 1 hour. In addition, sodium dodecyl sulfate (SDS) was added to the final concentration of 0.5%, and kept warm to 50°C for 6 hours to lead to bacteriolysis. To this bacteriolyzed solution, an equal amount of phenol/chloroform solution was added, gradually shaken at room temperature for 10 min, and then the total volume was centrifuged (5,000 x g, 20 min,10-12 °C ) to separate a fraction of supernatant. To this supernatant, sodium acetate was added to become at a concentration of 0.3 M, and then, double volume of ethanol was gradually added. DNA existing between a water layer and ethanol layer was taken up by glass rod, washed with 70% ethanol and then air-dried. To the DNA obtained, 5ml of a solution of 10mM Tris buffer (pH7.5)-1mM EDTA-2Na was added and allowed to stand over night at 4 °C followed by used for the subsequent experiments. (B) Cloning of a DNA fragment containing ADH gene and PDC gene from Zymomonas mobilis: PCR was conducted by using chromosomal DNA prepared in Example 1(A) as a template. In order to clone ADH gene and PDC gene In the PCR, the following each one pair of primers is synthesized by using "394 DNA/RNA synthesizer" (Applied Biosystems) and used. ADH gene-amplifying primer (a-1) 5'- tct cga get ctg tag ggt gag gtt ata get -3' (SEQ ID NO: 1) (b-1) 5'- etc tgg tac etc aag aca gga egg aaa ace -3' (SEQ ID NO: 2) Primer ( a-1) contains Sacl restriction site and primer ( b-1) contains Kpnl restriction site. PDC gene-amplifying primer * ' (a-2) 5'- tct cga att ctt gaa tat atg gag taa gca -3'(SEQ ID NO: 3) (b-2) 5'- tct cga get caa act aga gga get tgt taa -3'(SEQ ID NO: 4) Primer (a-2) contains EcoRI restriction site and primer (b-2) contains Sacl restriction site. Actually, PCR was conducted under the following conditions by using "DNA thermal cycler" (Perkin Elmer Cetus Co., Ltd.) and Taq DNA Polymerase/ TaKaRa Ex Taq (Takara Shuzo Co., Ltd.) as a reaction reagent. Reaction: (10X)PCR buffer 1.25mM dNTP mixture Template DNA (Content of DNA: less than 1 pM) 10 Ml 16 M' 10 Ml Two primers as described above *) 1 Ml (respectively) 0.5 Ml 61.5 M (Final concentration: 0.25MM) Recombinant Taq DNA Polymeraze Sterilized water *) When amplifying ADH gene, a combination of primers (a-1) and (b-1) was used, and when amplifying PDC gene a combination of primers (a-2) and (b-2) was used. The above components were mixed and 100 \i\ of the reaction was subjected to PCR. PCR Cvcle: Denaturation process: 94°C 60 sec 60 sec 52°C 1 20 sec 72°C Annealing process: Extension process: One cycle consisting of the above processes was repeated thirty cycles. Ten microliter of reaction prepared as stated above was subjected to an electrophoresis with 0.8% agalose gel, and about 1.2kb of a DNA fragment was detected for ADH gene, and about 1.7kb of a DNA fragment for PDC gene. Example 2 Preparation of a recombinant coryneform bacterium by ADH gene and PDC gene from Zymomonas mobilis. (A) Construction of shuttle vector: Five microliter of about 3.0 kb Hindll l-Hpal DNA fragment containing ORF1(rep)of plasmid pAM330 inherent to Brevibacterium lactofermentum ATCC13869 (Yamaguchi,R. et a I. T Agric. Biol. Chem. 50, 2771-2778 (1986), Japanese Patent Publication No. S58-67679) and 2 pi of plasmid pHSG398 (Takara Shuzo Co., Ltd.) which was cleaved with a restriction enzyme HindiII were mixed, to which each component of 1 (j| of T4 DNA ligase 10 x buffer, 1 unit of T4 DNA ligase was added, made 10 jj| with sterilized distilled water, and reacted at 15°C for 3 hours to combine. By using plasmid mixture thus obtained and calcium chloride method [ Journal of Molecular Biology, 53, 159(1970) ] , Escherichia coli JM109(Takara Shuzo Co., Ltd.) was transformed, and was plated onto medium (10 g of tryptone, 5 g of yeast extract, 5 g of NaCI, and 16 g of agar were dissolved in 1 L of distilled water) containing 50 mg of chloramphenicol. A strain grown on the medium was cultured in a liquid medium in a usual manner, plasmid DNA was extracted from the culture medium, and the plasmid was cleaved with a restriction enzyme to confirm the inserted fragment. As a result, in addition to 2.2 kb of DNA fragment of plasmid pHSG398 about 3.0 kb of the inserted DNA fragment was identified. This Escherichia coli-coryneform bacterium shuttle vector is referred to as pKP1. (B) Ligation of tac promoter with ADH gene and PDC gene: Five microliter of about 1\2 kb DNA fragment containing ADH gene from Zymomonas mobilis amplified in Example 1(B), and 2 |jl of plasmid pTrc99A(Pharmacia) containing tac promoter were respectively cleaved with restriction enzymes Sacl and Kpnl, the restriction enzymes were inactivated by treating at 70 °C for 10 min, and then both are combined, to which each component consisting of 1 pi of T4 DNA ligase 10 x buffer and 1 unit of T4 DNA ligase was added, made 10 pi with sterilized distilled water, and reacted at 15 °C for 3 hours to bind. This is used as ligation solution A. Similarly, 5 pi of about 1.7 kb DNA fragment containing PDC gene from Zymomonas mobilis amplified in Example 1(B) and 2 pi of plasmid pTrc99A(Pharmacia) containing tac promoter were respectively cleaved with restriction enzymes EcoRI and Sacl, and the restriction enzymes were inactivated by treating at 70 °C for 10 min and then both are combined, to which each component consisting of 1 ul of T4 DNA ligase 10 x bufferand 1 unit of T4 DNA ligase was added, made 10 ul with sterilized distilled water, and reacted at 15 °C for 3 hours to ligate. This is used as ligation solution B. By using each of two ligation solutions A and B, and calcium chloride method [Journal of Molecular Biology, 53, 159(1970)] , Escherichia coli JM109 (Takara Shuzo Co., Ltd.) was respectively transformed and was spread on a medium (10 g of tryptone, 5 g of yeast extract, 5 g of NaCI and 16 g of agar were dissolved in 1 L of distilled water) containing 50 mg of ampicillin. Each of strains grown on the media was cultured in a liquid medium, plasmid DNA was extracted from the culture medium, the plasmid was cleaved with restriction enzyme (Sad, Kpnl) for ligation solution A, and with restriction enzyme (EcoRI, Sad) for ligation solution B respectively, and the inserted fragment was confirmed. As a result, in addition to about 4.2 kb of DNA fragment from plasmid pTrc99A, an inserted DNA fragment with 1.2 kb in length was identified for ADH gene (ligation solution A), and an inserted fragment with 1.7 kb in length for PDC gene (ligation solution B). A plasmid containing ADH gene is referred to as pTrc99A-ADH, and a plasmid containing PDC gene is referred to as pTrc99A-PDC. Then, in order to prepare a DNA fragment wherein ADH gene was linked to tac promoter and a DNA fragment wherein PDC gene was linked to tac promoter by using PCR method in which plasmid pTrc99A-ADH and plasmid pTrc99A-PDC were used as a template, the following each one pair of primers was synthesized by using "394 DNA/RNA synthesizer" (Applied Biosystems). (a-3)5'- etc tag ate tec gac ate ata acg gtt ctg -3' (SEQ ID NO: 5) (b-3)5'- ctt etc tea tec gee aaa ca -3' (SEQ ID NO: 6) In addition, primer (a-3) contains Bglll restriction site. Actually, PCR was conducted under the following conditions by using "DNA thermal cycler" (Perkin Elmer Cetus Co., Ltd.) and Taq DNA Polymerase/ TaKaRa Ex Taq)(Takara Shuzo Co., Ltd.) as a reaction reagent. Reaction: (10X)PCR buffer 10 pi 1.25mM dNTP mixture 16 pi Template DNA*> 10 pi (Content of DNA: less than 1pM) Primers (a-3) and (b-3 ) as described above 1 pi (respectively) (Final concentration: 0.25pM) Recombinant Taq DNA Polymerase 0.5 pi Sterilized water 61.5 pi *) When amplifying a DNA fragment in which ADH gene is linked to tac promoter, plasmid pTrc99A-ADH was used as a template DNA, and when amplifying a DNA fragment in which PDC gene is linked to tac promoter, plasmid pTrc99A-PDC was used. The above components were mixed and 100 pi of the reaction was subjected to PCR. PCR Cycle: Denaturation: 94°C 60 sec Annealing: 52°C 60 sec Extension: 72°C 120 sec One cycle consisting of the above processes was repeated thirty cycles. Ten microliter of reaction prepared as stated above was subjected to electrophoresis on 0.8 % agalose gel, and about 1.4 kb of DNA fragment was detected for tac promoter-linked ADH gene, and about 1.9 kb of DNA fragment for tac promoter-linked PDC gene. (C) Insertion of a DNA fragment in which ADH gene is linked to tac promoter, and a DNA fragment in which PDC gene is linked to tac promoter into a shuttle vector: Five pi of reaction of about 1.9 kb of DNA fragment, wherein PDC gene was linked to tac promoter, of which amplified product was confirmed in the above Example 2(B) was completely cleaved with restriction enzymes Bglll and BamHI, and 2 pi of plasmid pKP1 prepared in the above Example 2 (A) was completely cleaved with restriction enzyme BamHI respectively. After the restriction enzymes were inactivated by treating at 70 °C for 10 min, both are combined, to which each component consisting of 1 |j| of T4 DNA ligase 10 x buffer and 1 unit of T4 DNA ligase was added, made 10 pi with sterilized distilled water, and reacted at 15 °C for 3 hours to bind. By using the obtained plasmid mixture and calcium chloride method [ Journal of Molecular Biology, 53, 159(1970) 3 , Escherichia coli JM109(Takara Shuzo Co., Ltd.) was transformed, and was spread on a medium (tryptone:1 Og, yeast extract:5g, NaCI:5 g and agar:16 g were dissolved in 1 L of distilled water) containing 50 mg of chloramphenicol. A strain grown on this media was cultured in a liquid medium, plasmid DNA was extracted from the culture medium, the plasmid was cleaved with restriction enzyme, and the inserted fragment was confirmed. As a result, in addition to about 5.2 kb of DNA fragment from plasmid pKP1 prepared in the above Example 2(A), an inserted DNA fragment with about 1.9 kb in length was identified. This plasmid is referred to as pKP1-PDC. This plasmid has only one BamHI restriction site. Five pi of reaction of about 1.4 kb of DNA fragment, wherein ADH gene was linked to tac promoter, of which amplified product was confirmed was completely cleaved with restriction enzymes Bglll and BamHI, and 2 ul of the above plasmid pKP1-PDC was completely cleaved with restriction enzyme BamHI respectively. After the restriction enzymes were inactivated by treating at 70 °C for 10 min, both are combined, to which each component consisting of 1 Ml of T4 DNA ligase 10 x buffer and 1 unit of T4 DNA ligase was added, made 10 ul with sterilized distilled water, and reacted at 15 °C for 3 hours to bind. By using the obtained plasmid mixture and calcium chloride method [ Journal of Molecular Biology, 53, 159(1970)] , Escherichia coli JM109(Takara Shuzo Co., Ltd.) was transformed and was spread on a medium (tryptone:10 g, yeast extract:5 g, NaCI:5 g and agar:16 g were dissolved in 1 L of distilled water) containing 50 mg of chloramphenicol. A strain grown on this media was cultured in a liquid medium, plasmid DNA was extracted from the culture medium, the plasmid was cleaved with restriction enzyme, and then, the inserted fragment was confirmed. As a result, in addition to about 7.1 kb of DNA fragment from plasmid pKP1 prepared in the above Example 2(A), an inserted DNA fragment with about 1.4 kb in length was identified. This plasmid was named pKP1-ADH (SEQ ID NO: 7). (D) Transformation of coryneform bacterium: The plasmid was introduced into Corynebacterium glutamicum ATCC13032 by using electrical pulse method (Y. Kurusu, et al., Agric. Biol. Chem. 54: 443-447. 1990 and A. A. Vertes, et al., Res. Microbiol. 144:181-185. 1993). A transformant thus obtained, Corynebacterium glutamicum pKP1-PDC-ADH/13032 was deposited with National Institute of Bioscience and Human Technology (1-3 Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan) on June 6, 2000 (accession No. PERM P-17887), and then transferred to the International Deposit under Budapest Treaty on May 31, 2001 (International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan, accession No. FERM BP-7621). Example 3 Ethanol production by Corynebacterium glutamicum ATCC13032 transformant (Corynebacterium glutamicum pKP1-PDC-ADH/13032) A medium consisting of urea:40 g, (NH4)2S04:140 g, KH2PO4:5.0 g, K2HPO4:5.0 g, MgS04 • 7H2O:5.0 g, FeS04 • 7H2O:200 mg, MnS04 • nH2O:200mg, D-biotin:2000 pg, thiamine hydrochloride: 1000 pg, yeast extract: 10 g, casamino acids: 10 g and distilled water : 10 l(pH6.6) was poured in 500ml portion into 2 I Erlenmeyer flask, sterilized at 120 °C for 15 min, to which 40 ml of an sterilized aqueous solution of 50 % glucose was added. To the medium, Corynebacterium glutamicum strain which was transformed by introducing the above pKP1-PDC-ADH plasmid(pKP1-PDC-ADH/13032)was inoculated and cultured at 33°C for 24 hours with stirring (aerobic culture). After completion of culture, it was centrifuged (8000 g, 20 min) to herveste the bacterial cells. The total volume of the obtained bacterial cells were subjected to the following reactions. Five hundred ml of reaction consisting of (NH4)2S04:23 g, KH2PO4:0.5g, K2HPO4:0.5 g, MgS04 * 7H2O:0.5 g,FeS04 • 7H2O:20 mg, MnS04 • nH2O:20mg, D-biotin:200 pg, thiamine hydrochloride: 100 pg, sodium carbonate 20 g, distilled water :1000mlwas poured into 1 L jar fermentor and the above bacterial cells and 50 % glucose solution 120ml were added, which was reacted at 30 °C with slowly stirring (200 rpm) under sealed condition (anaerobic reaction). After 2 and 4 hours, the respective reactions were centrifuged (8000 rpm, 15 min, at 4 °C), and each of supernatants thus obtained was then subjected to gas chromatography to be revealed the ethanol production at the concentration of 3.79 and 6.96(g ethanol/l), respectively. These results are shown in the following Table 1. Table 1 Reaction time(hr) 2 4 Concentration of produced ethanol (g ethanol/l) 3.79 6.96 Mean rate of ethanol production (g ethanol/l/hr) 1.89 1.74 Example 4 Ethanol production by using Brevibacterium lactofermentum ATCC13869 transformant (pKP1 -PDC-ADH/1 3869) Piasmid pKP1-PDC-ADH prepared according to the method of Example 2 (C) was introduced into Brevibacterium lactofermentum ATCC13869 in a similar manner as the method of Example 2 (D). The obtained transformant, Brevibacterium lactofermentum pKP1-PDC-ADH/13869 was deposited with National Institute of Bioscience and Human Technology (1-3 Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan) on June 6, 2000 (accession No. FERM P-17888), and then transferred to the International Deposit under Budapest' Treaty on May 31, 2001 (International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan, accession No, FERM BP-7622). In a similar manner as the method of Example 3, the obtained coryneform * bacterium transformant was aerobically cultured, and subjected to a reaction to anaerobically produce ethanol. After 4 hours, the concentration of ethanol in a reactor was 4.21(g ethanol/l). Thus, the mean rate of ethanol production is 1.05(g ethanol/l/hr). EFFECT OF THE INVENTION According to the present invention, reaction of an aerobic coryneform bacterium transformed with ADH gene and PDC gene under anaerobic condition allows efficiently producing ethanol at a high productivity. SEQUENCE LISTING FREE TEXT SEQ ID NO: 1: A primer for amplifying ADH gene SEQ ID NO: 2: A primer for amplifying ADH gene SEQ ID NO: 3: A primer for amplifying PDC gene SEQ ID NO: 4: A primer for amplifying PDC gene SEQ ID NO: 5: A primer for amplifying ADH gene linking with tac promoter. SEQ ID NO: 6: A primer for amplifying PDC gene linking with tac promoter. SEQ ID NO: 7: E.coli/coryneform bacteria shuttle vector having PDC gene linking with tac promoter ADH gene linking with tac promoter. CLAIMS: 1. A process for producing ethanol using a coryneform bacterium, which has been transformed by a DNA containing a gene expressing pyruvate decarboxylase activity and, if desired, a gene expressing alcohol dehydrogenase activity under a regulatory sequence allowing for the expression, characterized in that ethanol is prepared under conditions wherein the transformed coryneform bacterium does not substantially proliferate. 2. An expression vector which is used for a process as set forth in Claim I to transform a coryneform bacterium, into which a DNA containing a gene expressing pyruvate decarboxylase activity and, if desired, a gene expressing alcohol dehydrogenase activity is incorporated under a regulatory sequence allowing for the expression. 3. An expression vector as set forth in Claim 2 wherein the vector is plasmid pKP1 -PDC-ADH. 4. A coryneform bacterium transformed with an expression vector as set forth in Claim 2 or 3. 5.An coryneform bacterium as set forth in Claim 4 wherein the bacterium is Coryne bacterium glutamicum pKP1-PDC-ADH/13032. 6. An coryneform bacterium as set forth in Claim 4 wherein the bacterium is Brevibacterium lactofermentum pKP1-PDC-ADH/13869. 8. A process for producing ethanol substantially as herein described and exemplified. 9. An coryneform bacterium substantially as herein described and exemplified.

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